This invention relates to a novel method for photodepositing a viewing-screen structure for a CRT (cathode-ray tube), particularly for a multibeam color display tube. The screen structure can be, for example, a light-absorbing matrix or luminescent elements of the viewing screen.
A color television tube, which is a type of CRT, comprises an evacuated glass envelope including a faceplate panel having a viewing window, a viewing screen on the inside surface of the window, and means for selectively exciting elements of the screen to luminescence. In one type of picture tube, the viewing screen is comprised of interlaced elements having different light-emission characteristics. Also, the tube includes an apertured shadow mask closely spaced from the viewing screen. The mask is part of the means for selectively exciting the viewing screen, and also is used as a photographic master for depositing the screen structure.
A typical process for fabricating the screen structure includes three photographic exposures, one for defining the elements of each of three different luminescent fields. Each exposure involves projecting a light field from a light source, through a light-refracting lens, an IC (intensity-correcting) filter, and a photographic master incident on a photosensitive layer that is supported on the inside surface of the viewing window. The exposures differ in that the panel is displaced laterally for each exposure relative to the axis of the lens.
Because of the optical characteristics of the system, the brightness of the unfiltered light field drops off from center to edge. To compensate for this, the transmission of the IC filter increases from center to edge. And, because it is desirable for screen elements to decrease in size from center to edge, the filter produces a brightness profile at the photosensitive layer which produces the desired distribution of screen-element sizes. The filtered light field may drop off in brightness from center to edge, but not as sharply as for the unfiltered light field. And, the brightness of the light field varies according to prescribed profiles. One particularly useful optical IC filter that can be used for this purpose is disclosed in U.S. Pat. No. 4,132,470 to H. F. van Heek issued Jan. 2, 1979. That filter, which is referred to in the art as a half-tone line-pattern IC filter, includes a transparent plate and a multiplicity of opaque, substantially-parallel, spaced stripes or lines. The filter has local regions of prescribed optical transmissions produced by variations in the widths of the stripes in those regions. That IC filter can be made with an optical drawing machine by drawing parallel spaced stripes of substantially uniform pitch therebetween but of varying widths according to a mathematical prescription. Working filters are then made by contact printing with the optically-drawn masters.
The above-described IC filter can be made reliably with lines having a 15-mil pitch and a minimum width of about 1.5 mils, whereby a maximum transmission of about 90% is realized. Where a relatively-long exposure is required for photodepositing a CRT screen structure, it is desirable to use a filter with a higher maximum transmission in order to shorten the required exposure time. Also, it is desirable to employ a filter having opaque elements that are arranged along lines with smaller pitch therebetween in order to reduce the vestige of filter line structure in the CRT viewing screen structure.